Norovirus Outbreak Suggests Change Needed

A norovirus outbreak on a cruise ship resulted in a 15% attack rate despite early detection and aggressive measures to contain illness, the CDC reported.

The outbreak, in January 2009, sickened 236 of the 1,532 passengers who completed a CDC questionnaire during the cruise, according to Mary Wikswo, MPH, and colleagues at the CDC.

But of those, only 60% had sought medical care from the ship's infirmary, Wikswo and colleagues reported online in Clinical Infectious Diseases.

One implication of their investigation, the researchers concluded, is that the threshold for declaring an outbreak may need to be adjusted to account for poor voluntary reporting of acute gastroenteritis.

Under the CDC's Vessel Sanitation Program, an outbreak of acute gastroenteritis is declared when at least 3% of passengers report illness. That threshold was met aboard the vessel -- identified as Cruise Ship X -- within three days of the start of the 14-day voyage, Wikswo and colleagues reported.

CDC investigators boarded the vessel, took samples from some passengers, tested sanitation practices, and developed a questionnaire that was distributed to all 1,842 passengers. The questionnaire was filled out by 83.2%, the researchers reported.

The 859 crew members were excluded from the questionnaire because only seven of them had become ill, an attack rate of less than 1%, possibly because most had limited contact with passengers.

The investigators defined a case of acute gastroenteritis as more than three episodes of loose or watery stools within a 24-hour period or any vomiting accompanied by at least one episode of loose or watery stools, fever, abdominal cramping, myalgia, or headache.

The period of interest was from Jan. 1 through Jan. 14, or three days before the vessel departed to three days before the end of the voyage.

Wikswo and colleagues reported:

Case passengers were more likely to have an ill cabin mate (RR 3.0, P<0.01).

They were also more likely to have witnessed an incident in which an embarking passenger vomited in the guest services office on the vessel's deck 5 (RR 2.8, P<0.01).

More than 90% of passengers reported increased hand hygiene practices following the outbreak, and 38% of ill passengers and 11% of well passengers cut down on public activities.

Of 14 stool samples tested, 12 were positive for norovirus by polymerase chain reaction testing and five were confirmed by sequence analysis.

Although few passengers witnessed the vomiting incident, it is possible it was one of the seeds that sparked the outbreak, Wikswo and colleagues said, either through ingestion of aerosolized particles of vomitus or through contaminated surfaces.

All told, the ship's records showed eight such episodes in public areas, all reported having been cleaned within 30 minutes. But, the researchers reported, the material used to absorb and bind the vomit was not used as intended.

One of the implications of the finding, Wikswo said in a statement, is that "cruise line personnel should discourage ill passengers from boarding their ships."

Once they board, she added, "passengers and crew who become ill should report to the ship's medical center as soon as possible."

The ship's crew was quick to undertake several measures to reduce the spread of illness, the researchers noted, including closing self-service sections of the dining areas and isolating case passengers and their cabin mates.

Despite that, the investigators found several violations that might have contributed to the outbreak, including toilets being out of hand soap and paper towels and dishwashing machines that did not do an adequate sanitation of utensils.

Another possibility, they noted, is that cruise lines could offer incentives to encourage passengers to report illness in order to allow more timely and aggressive control measures.

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